Sure, here's an example of an Emoji Jumpscare game implemented as a single HTML file with JavaScript:
In this game, the player controls an emoji character by tapping the screen. The player jumps when tapped. There is a chance (10%) for a jumpscare to appear randomly and momentarily on the screen, triggered by the jumpScare() function.
Please note that since this is a simple example, the jumpscare is only represented by a black overlay on the screen for a brief moment. You can modify the `showJumpScare()` function to add actual jumpscare images or animations as per your requirement.